This invention concerns improvements in or relating to ultrasonic motors.
Ultrasonic motors produce high torque with a low maximum speed and have a high power and torque density. There are a large number of current designs but almost all designs require precision construction methods and so are expensive to make.
The present invention is intended to remedy this drawback. More particularly, the present invention seeks to provide an ultrasonic motor that can be made simply, with few components and at low cost.
According to the broadest aspect of the present invention there is provided an ultrasonic motor in which oscillating vibrations are converted into rotary motion through frictional contact at an interface between relatively rotatable components of the motor wherein one of the components comprises a disc of electro-active material and at least one flextensional displacement amplifier diaphragm for converting radial vibrations of the disc into oscillating vibrations of the or each diaphragm perpendicular to the plane of the disc.
The invented ultrasonic motor utilises one or two flextensional diaphragms as displacement amplifiers attached to the disc. These greatly increase the amplitude of vibrations and convert the radial vibrations of the disc into vibrations of the or each diaphragm perpendicular to the disc plane. Such flextensional amplifiers are used for sonar applications and for increasing the displacement given by electro-active displacement transducers. The innovative aspect of this design is that it converts the vibrations to rotational movement by a frictional push and release method.
Furthermore, the invented ultrasonic motor requires only a single electrical supply phase to drive it and has a positive braking torque with no power applied. The excitation of the disc in a radial mode gives a high electromechanical coupling factor producing an efficient and high power density ultrasonic motor. The high coupling factor also allows the use of a simple ac electrical drive without feedback to control frequency since the resonant bandwidth of the motor is large.
Preferably, the frictional contact is provided by a plurality of drive elements at the contact interface. In a preferred arrangement, the drive elements comprise elastic fins on one of the diaphragm(s) and an opposed rotor which are pressed into frictional contact with the other of the diaphragm and rotor. For example, the diaphragm and rotor may be urged towards each other by a spring force. Alternatively, the diaphragm and rotator may be urged towards each other by a magnetic attraction force.
Furthermore, this method of producing a rotary motion from a vibrating surface relies on a frictional contact between the tips of the elastic fins and the relatively moving surface, which is maintained with a positive contact pressure supplied by a spring or magnetic attraction. The fins impart a small displacement on the rotor, of the order of a few microns, on each upward stroke of the diaphragm then the fin retreats to return to its original position on the downward travel of the diaphragm""s surface.